The present invention relates to an air-fuel ratio controller of an engine having an air-fuel ratio correction coefficient calculating means and, more particularly, to an air-fuel ratio controller of an engine having an air-fuel ratio correction coefficient calculating means coping with a time-lag in detection of an air-fuel ratio detected by a wide range air-fuel ratio sensor.
In a case where an oxygen concentration in an exhaust gas of an engine is detected by an air-fuel ratio sensor, and an air-fuel ratio of a mixed gas to be supplied to the engine is controlled to a preset value, for example, to a value near the theoretical air-fuel ratio by feeding back the detected value, response of the air-fuel ratio correction control is improved by calculating an air-fuel ratio correction coefficient in taking into consideration a time lag from the time when fuel is injected by an injector and burned to the time when an oxygen concentration is detected. Such a technology is disclosed, for example, in Japanese Patent Application Laid-Open No.5-288105. As a means for solving the problem of the time lag described above, a technology of improving control response at normal operation by correcting a preset air-fuel ratio using a model of a dead time and a first-order time lag constant and a technology of improving control response at transient operation by correcting an air-fuel ratio control parameter changing a throttle opening are proposed.
Further, a technology for solving the problem of the time lag in response of correcting air-fuel ratio is proposed in Japanese Patent Application Laid-Open No.8-74624. The technology is that in an air-fuel ratio controller for an engine, when an air-fuel ratio correction coefficient for setting a fuel amount supplied to the engine is calculated, the problem of the time lag in response of correcting air-fuel ratio is solved by performing PI control (proportional control and integral control) based on a detected value of oxygen concentration detected by an O.sub.2 sensor, that is, by performing proportional control (proportional correction) based on the detected value and then performing integral control (integral correction) by increasing the integral coefficient corresponding to elapsed time.
The former conventional technology described above has disadvantages in that a large number of man-hours required in matching work such as model setting and sharing among parameters when constant matching work is performed for each system to be adjusted, and that an error is difficult to be corrected when the error is produced in the model due to deterioration over time.
The latter conventional technology described above has a disadvantage in that the method can not calculate any air-fuel ratio correction coefficient for PI control based on an air-fuel ratio value (linear detected value) detected by a wide range air-fuel ratio sensor because the method is PI control (proportional control and integral control) based on a detected value (ON-OFF detected value) of oxygen concentration detected by an O.sub.2 sensor.
Therefore, even if the PI control based on the air-fuel ratio value detected by the O.sub.2 sensor is applied to an air-fuel ratio correction coefficient calculating means using the wide range air-fuel ratio sensor, the following problems occur. That is, since an output from the wide range air-fuel ratio sensor has a time lag to the change in an air-fuel ratio supplied to the engine and an amplitude of the output is averaged to become small, as shown in FIG. 8(a), a time lag as shown by a dashed line in FIG. 8(b) occurs when the air-fuel ratio control by PI control is performed. As a result, an appropriate air-fuel ratio correction coefficient can not be calculated, and response of the air-fuel ratio correction control is deteriorated, and problems occur in the engine output and the exhaust gas cleaning. It can be considered that a means for solving the time lag in control is to increase a control gain. However, in this case, a problem such as increase of overshoot or occurrence of oscillation may be occur.